Thermoplastic packaging films made of vinylidene chloride copolymer, hereinafter referred to generally as "saran", have been used for a long number of years to package food products which include cheese, fresh and processed meats, and a wide variety of other food and non-food items. Examples of such films are disclosed in U.S. Pat. No. 2,919,059 which issued in December 1959 to Arthur F. Sporka. This patent discloses laminate films of cellophane-saran-polyethylene and polyethylene-saran-polyethylene which are made by treating the polyethylene film surface to enhance its cling sheer strength so that a laminate or multi-layer film can be made by using the inherent attractive forces in the face-to-face surfaces of adjoining plies of pre-formed film.
A later and more satisfactory method of producing a multi-layer film having a layer of saran is disclosed in U.S. Pat. No. 4,112,181 which issued on Sept. 5, 1978 to William G. Baird, Jr. et al. In this patent a method of coextruding a tubular film is described wherein the walls of the tube have at least three layers, a center layer being a saran layer. The tubular film is subsequently biaxially oriented by the trapped bubble technique.
Another satisfactory method of producing a multi-layer saran film is disclosed in U.S. Pat. No. 3,741,253 which issued on June 26, 1973 to Harri J. Brax et al, which specifically discloses a multi-layer, biaxially oriented film having a saran barrier layer. This film is made by an extrusion coating process in which a substrate of polyethylene or ethylene vinyl acetate copolymer is coextruded in the form of a tube, cross-linked by irradiation, inflated into a tubing, a layer of saran extrusion coated onto the inflated tubing, and then another layer of ethylene-vinyl acetate copolymer is coated onto the saran. After cooling, this multi-layer tubular structure is flattened and rolled up. At a later time, the tube is inflated, sent through a hot water bath where it is heated to its orientation temperature, and as it is drawn out of the the bath, it is inflated into a bubble thereby biaxially orienting the film. The bubble is rapidly cooled to set the orientation. This process produces a shrinkable barrier film with low oxygen permeability. Also, the advantages of a cross-linked film are provided without subjecting the saran layer to irradiation which tends to degrade saran.
The barrier layer in the above mentioned patent to Brax et al is a plasticized copolymer of vinylidene chloride and vinyl chloride. The copolymer is a mixture of 10% suspension polymerized and 90% emulsion polymerized copolymer. The emulsion polymerized copolymer comprises about 70% vinylidene chloride and 30% vinyl chloride and the suspension polymerized copolymer comprises about 80% vinylidene chloride and 20% vinyl chloride. Such copolymers are obtainable from the Dow Chemical Company of Midland, Mich.
In order to be successfully extruded at commercial rates the foregoing described mixture of vinylidene chloride copolymers must be stabilized and plasticized. A successful stabilizer-plasticizer combination is epichlorohydrin/bisphenol, an epoxy resin sold as EPON resin 828 by the Shell Chemical Company, and 2-ethyl hexyl diphenyl phosphate sold as Santicizer-141 by Monsanto Chemical Co. Other known stabilizers include epoxidized linseed oil and epoxidized soybean oil and citrates. A quite successful and satisfactory plasticizer package is made using approximately 4% of the Epon 828 and approximately 2% of the Santicizer-141 in the foregoing described mixture of vinylidene chloride copolymers.
In Canadian Pat. No. 968,689, which was issued on June 5, 1975 to Mario Gillio-Tos et al, the effect of plasticizers on the barrier properties of a saran barrier layer in a multi-layer thermoplastic packaging film is described. First, the Gillio-Tos et al patent discloses that homopolymers of vinylidene chloride cannot be converted into film by conventional extrusion techniques because they decompose very rapidly at the temperature of extrusion. Second, by copolymerizing vinylidene chloride with minor amounts of one or more other monomers such as vinyl chloride, methyl acrylate, etc. it is possible to produce copolymers which, when mixed with suitable plasticizers, can be extruded into films which can be oriented by stretching to give shrinkable film. The oriented films are heat shrinkable and are widely used for packaging purposes, particularly for packaging food. As stated, vinylidene chloride copolymers need to be plasticized so that they can be satisfactorily extruded and stretched into oriented films. The greater the proportion of plasticizer the easier the polymer is to extrude and orient and the better the abuse resistance of the final product. On the other hand, the oxygen permeability of the final product also increases with increasing plasticizer content and for many purposes, especially packaging food, it is vital that the oxygen permeability should be low. In recent years, the requirements of the packaging industry have become more and more demanding and for current commercial practices permeability below 100 cc/24 hours/m.sup.2 /atmosphere is expected and a permeability below 50 is highly desirable.
Accordingly, it is an object of the present invention to provide a vinylidene chloride copolymer composition which can be extruded with reduced plasticizer content and, hence, lowered and improved oxygen permeability.
By lowering the oxygen and gas permeability the thickness of barrier layers and, hence, the quantity of saran required for a barrier film is reduced. Also since the thickness of saran layers can be reduced, it is another object of the present invention to provide a film wherein the saran layer is thick enough to be an effective gas barrier but thin enough so that it is not adversely affected to any significant extent by irradiation used to cross-link layers which are cross-linkable and adjacent to the saran layer in a multi-layer film.
Still another object of the present invention is to improve the thermal stability of vinylidene chloride copolymers and to lessen their tendency to degrade while being extruded.
Among the plasticizers for thermoplastics which are listed in general articles and in literature references is glycerol or glycerin. In fact, in "Whittington's Dictionary of Plastics", Technomic Publishing Company, Inc., West Port, Conn., 1978, at page 152, glycerol is listed and the following statement about it is made: "Its uses in the plastics industry include the manufacture of alkyd resins (esters of glycerol and phthalic anhydride); the plasticization of cellophane; and the production of urethane polymers." Glycerin has also been used as a plasticizer for ethylene-vinyl alcohol compolymers and glycerol compounds such as glycerol mono-oleate have been used as heat stabilizing additives for vinylidene halide polymers as described in U.S. Pat. No. 4,274,999 issued on June 23, 1981 to Joseph W. Burley et al. Also in U.S. Pat. No. 4,115,334 which issued on Sept. 19, 1978 to C.W. Gerow an antistatic composition of a poly basic acid with a mono- or di- glyceride and a vinylidene chloride copolymer as an antistatic agent is disclosed. While a glycerol derivative has been used in the specific instances mentioned, a unique combination has been disclosed and thus, another object of the present invention is to provide an improved stabilizer-plasticizer combination for saran which employs glycerol alone.